This invention relates to the field of pulse-width-modulated (PWM) circuits, and provides a circuit for applying a voltage, and thereby a current, to a load, wherein the waveform representing the current flowing through the load is substantially free of distortion.
Examples of PWM circuits are shown in U.S. Pat. Nos. 5,070,292, 5,081,409, 5,379,209, and 5,365,422. The disclosures of all of the latter patents are hereby incorporated by reference into this specification. These patents give examples of circuits in which a series of pulses controls a set of electronic switches which selectively connect a power supply to a load. The load can be an electric motor, or a coil used to produce a magnetic field, or some other load. PWM circuits often have the form of an H-bridge, which comprises four switches, arranged symmetrically around the load, the switches providing the desired paths between a power supply and the load.
In theory, at any given instant, two of the four switches of an H-bridge are "on" and the other two switches are "off", and the currents flow exactly in the desired paths. In practice, the switches are not ideal, and their states cannot be changed instantaneously. In fact, each switch contains a parasitic diode, formed by the source and drain of a field effect transistor, for example, and this parasitic diode can conduct current in the reverse direction, regardless of whether the switch is "on" or "off". Furthermore, when the diode has been conducting, it takes a finite amount of time to stop conducting (i.e. to turn "off"). Due to these non-ideal properties of the switches, there are moments at which current may flow, with little or no resistance, from the power supply, through two switches (bypassing the load) and back to the power supply, creating a momentary short circuit. This condition is known as "cross-conduction", and the current is known as "shoot through" current. In the case of diode recovery, the condition is known as "recovery current".
Prior art circuits have overcome the problem of shoot through current by providing a "dead time" in the PWM circuit. That is, the switch that would otherwise cause a momentary short circuit is prevented from closing by delaying the arrival of a leading edge of a pulse. However, such artificial dead time introduces considerable non-linearity into the system. The waveform of the current flowing through the load becomes distorted, and the result is unsuitable for applications requiring high fidelity, such as audio amplification.
The present invention provides an improved PWM circuit in which the current flowing through the load is substantially undistorted, but in which the problem of shoot through current is reduced or eliminated.